In order to improve the image quality of movies, it is important to design projection lenses with higher MTF (Modulation Transfer Function) values. Some enhanced optical designs have been implemented in the last twenty years which improved the quality of movies. FIG. 1 shows a cross section of an Ultra Star HD lens with a focal length of 85 mm, which represents the current state of the art. See "Press release Fernseh-Und Kinotechnik 10/88." An improvement in the MTF of this design is shown in FIG. 2a comparing it to the MTF of a competing product, the Cinelux-Ultra MC lens. The MTF of the Ultra Star HD lens across the image field for three spatial frequencies, 30 lines/mm, 50 lines/mm, and 70 lines/mm is illustrated in FIG. 2b. While these newer designs have had a significant impact on the image quality of movies, progress can still be made to further push the state of the art of movie projection lens optics.
The primary limiting factor in projector lens optics is secondary color aberration, both axial and lateral. Primary axial color aberration is corrected when light at the ends of the visible spectrum, red and blue, are brought to the same focus. Secondary axial color aberration is the focus separation between the light in the center of the visible spectrum and the light at the ends of the visible spectrum. Spherochromatism, the variation of spherical aberration with wavelength limits the performance of state of the art lenses, as does secondary lateral color aberration, which is the variation of image size between images in the center of the visible spectrum and the ends of the visible spectrum. Secondary chromatic aberration is notoriously difficult and expensive to correct because exotic glasses are required if a conventional all-refractive optical system is used. Glass with the required optical properties is expensive, and is typically difficult to manufacture.
It has been known in the art that diffractive lenses can reduce chromatic aberrations. The earliest references on this subject were applied to holographic lenses, a particular type of diffractive lens. See G. M. Morris, "Diffraction Theory for an Achromatic Fourier Transform," Appl. Opt. 20, 2017 (1981); T. Stone and N. George, "Hybrid Singlet Arbitrarily Dispersive Element," J. Opt. Soc. Am. A4(13), 77(1987); T. Stone and N. George, "Hybrid Diffractive-refractive Lenses and Achromats," 27(14), 2969(1988). Diffractive lenses, with an effective V number of -3.45 in the visible, have large dispersions and are opposite to that of refractive glass lenses, enabling compensation of refractive lens dispersions by using weak diffractive lens powers.
Many patents have issued taking advantage of this fact, including U.S. Pat. Nos. 5,923,479, 5,883,744, 5,880,879, and 5,790,321. For example, U.S. Pat. No. 5,923,479 teaches how to use one diffractive lens to reduce both axial and lateral color aberration for a retro-focus type lens. A retro-focus lens has a front negative group and a positive rear group producing a back focus greater than the lens focal length. In this patent the diffractive lens must meet certain conditions and be placed within the rear positive group. None of these references disclose the use of more than one diffractive optic to correct both axial and lateral color simultaneously, particularly both secondary axial and lateral chromatic aberrations.
The use of diffractive lenses in a movie projection lens is particularly beneficial to correct color aberration for a number of reasons. Movie projection lenses must withstand very hot environments due to the high power light sources used in movie projectors. The high power is needed because of the high magnifications required in movie theatres. It is not unusual to have magnifications of over three hundred times. The high temperatures reached by the projection optics has an impact not only on the glass choices, but the fact that no known optical cement can withstand such temperatures. This eliminates the optical design power of using cemented doublets. For example, it would normally be beneficial to use cemented doublets in the outer lens groups to correct lateral color aberration. The inability to use cemented doublets to correct chromatic aberration can be compensated through the use of diffractive optics.
One of the negative aspects of using diffractive optics is that they generally produce more flare light than refractive lenses. This is particularly true for diffractive optics made by the binary method in which approximations to smooth surfaces are made in a stair step fashion. FIG. 3a shows the continuous phase introduced into the optical wavefront by a refractive surface. Because the effects of phase repeat every 2#, or one wavelength of oscillation, it is possible to break up the phase in pieces every 2#, or one wavelength as shown in FIG. 3b. This type of surface is known as a kinoform. See L. B. Lesem and P. M. Hirsch, "The Kinoform: A New Wavefront Reconstruction Device," IBM Journal of Research and Development, Vol. 13, pp. 150-155(1969). A true binary version of the surface is shown in FIG. 3c, which has two phase levels, the surface being broken every half wavelength. A four-level binary approximation is shown in FIG. 3d in which the surface is broken every quarter wavelength. FIG. 3e is an eight-level binary version. Table 1 lists the diffraction efficiency for the various binary surfaces. The remaining light strikes the image in undesirable regions and becomes flare light. As can be seen, the more levels used in the binary diffractive optic, the more efficient it becomes with the kinoform being the most efficient.
TABLE I Number of phase Diffraction levels Figure efficiency 2 2c 40.5% 4 2d 81.1% 8 2e 95.0% 16 98.7%
With the movie film projection, contrast ratios above 600 on theater screens are at the upper end of the capabilities of current theaters for film with a density range between D.sub.max and D.sub.min of 3.2 having an inherent capability of almost 1600 to one contrast ratio. The reduction is caused by flare light. Flare light from well-made kinoform diffractive lenses will not be easily detected under normal conditions. Also, because the scenes in movies are continually changing, the presence of small, additional flare on top of that already present will not be easily observable.
It is desirable to use diffractive lenses in a movie projector lens while limiting secondary axial and secondary lateral color aberration.